additional_settings should override the to_dict method

See for example [here](https://github.com/cloudify-cosmo/cloudify-gcp-plugin/blob/master/cloudify_gcp/compute/instance.py#L117) and [here](https://github.com/cloudify-cosmo/cloudify-gcp-plugin/blob/master/cloudify_gcp/compute/instance.py#L363).